Alloy type thermal fuse and material for a thermal fuse element

ABSTRACT

An alloy type thermal fuse of an operating temperature of 75 to 120° C. is provided in which a fuse element of a Bi—In—Sn alloy is used, excellent aging and heat cycle resistances for a long term can be ensured, and satisfactory operating characteristic can be ensured.  
     A material for a thermal fuse element has an alloy composition in which In is 15% or larger and smaller than 37%, Sn is 5% or larger and 28% or smaller, and balance Bi, and in which, with respect to each of reference points of ternary Bi—In—Sn eutectic points of 57.5% Bi-25.2% In-17.3% Sn and 54.0% Bi-29.7% In-16.3% Sn, a range of ±2% Bi, ±1% In, and ±1% Sn is excluded.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a material for a Bi—In—Sn alloytype thermal fuse in which the operating temperature belongs to a rangeof 75 to 120° C., and also to such a thermal type fuse element.

[0003] An alloy type thermal fuse is widely used as a thermo-protectorfor an electrical appliance, a circuit element, or the like.

[0004] Such an alloy type thermal fuse has a configuration in which analloy of a predetermined melting point is used as a fuse element, thefuse element is bonded between a pair of lead conductors, a flux isapplied to the fuse element, and the flux-applied fuse element is sealedby an insulator.

[0005] The alloy type thermal fuse has the following operationmechanism.

[0006] The alloy type thermal fuse is disposed so as to thermallycontact an electrical appliance or a circuit element which is to beprotected. When the electrical appliance or the circuit element iscaused to generate heat by any abnormality, the fuse element alloy ofthe thermal fuse is melted by the generated heat, and the molten alloyis divided and spheroidized because of the wettability with respect tothe lead conductors or electrodes under the coexistence with theactivated flux that has already melted. The power supply is finallyinterrupted as a result of advancement of the spheroid division. Thetemperature of the appliance is lowered by the power supplyinterruption, and the divided molten alloys are solidified, whereby thenon-return cut-off operation is completed.

[0007] 2. Description of the Prior Art

[0008] Conventionally, a technique in which an alloy composition havinga narrow solid-liquid coexisting region between the solidus and liquidustemperatures, and ideally a eutectic composition is used as such a fuseelement is usually employed, so that the fuse element is fused off atapproximately the liquidus temperature (in a eutectic composition, thesolidus temperature is equal to the liquidus temperature). In a fuseelement having an alloy composition in which there is a solid-liquidcoexisting region, namely, there is the possibility that the fuseelement is fused off at an uncertain temperature in the solid-liquidcoexisting region. When an alloy composition has a wide solid-liquidcoexisting region, the uncertain temperature width in which a fuseelement is fused off in the solid-liquid coexisting region becomeslarge, and the operating temperature is largely dispersed. In order toreduce the dispersion, therefore, the technique in which an alloycomposition having a narrow solid-liquid coexisting region between thesolidus and liquidus temperatures, or ideally a eutectic composition isused is usually employed.

[0009] In a high-energy density secondary battery which is generallyused as a power source of a portable telephone, a notebook personalcomputer, or a like portable electronic apparatus, such as a lithium-ionbattery or a lithium polymer battery, a large amount of heat isgenerated in an abnormal state. Therefore, a thermal fuse is attached toa battery pack, and, when a battery reaches a dangerous temperature, thethermal fuse operates to prevent abnormal heat generation fromoccurring. The operating temperature of such a thermal fuse is set to bewithin a range of 75 to 120° C.

[0010] Because of increased awareness of environment conservation, thetrend to prohibit the use of materials harmful to a living body isrecently growing, and also an element for such a thermal fuse isstrongly requested not to contain a harmful element (Pb, Cd, Hg, Tl,etc.).

[0011] As an alloy composition which can satisfy the requirement, knownis a Bi—In—Sn system. Conventionally, the following thermal fuses whichhave an alloy composition of Bi—In—Sn, and which satisfy the requirementof an operating temperature of 75 to 120° C. are known: a thermal fusein which a fuse element has an alloy composition of 47 to 49% Sn, 51 to53% In, and an adequate amount of Bi, and which has an operatingtemperature of 105 to 115° C. (Japanese Patent Application Laying-OpenNo. 56-114237); that in which a fuse element has an alloy composition of42 to 53% In, 40 to 46% Sn, and 7 to 12% Bi, and which has an operatingtemperature of 95 to 105° C. (Japanese Patent Application Laying-OpenNo. 2001-266724); that in which a fuse element has an alloy compositionof 51 to 53% In, 42 to 44% Sn, and 4 to 6% Bi, and which has anoperating temperature of 107 to 113° C. (Japanese Patent ApplicationLaying-Open No. 59-8229); that in which a fuse element has an alloycomposition of 1 to 15% Sn, 20 to 33% Bi, and the balance In, and whichhas an operating temperature of 75 to 100° C. (Japanese PatentApplication Laying-Open No. 2001-325867); and that in which a fuseelement has an alloy composition of 0.3 to 1.5% Sn, 51 to 54% In, andthe balance Bi, and which has an operating temperature of 86 to 89° C.(Japanese Patent Application Laying-Open No. 6-325670). Furthermore, athermal fuse is known in which a fuse element has an alloy compositionof a Bi—In system not containing Sn and of 45 to 55% Bi and the balanceIn, and which has an operating temperature of 85 to 95° C. (JapanesePatent Application Laying-Open No. 2002-150906). Moreover, an In—Sneutectic alloy (52% In, 48% Sn) having a melting point of 119° C. may becontemplated to be used as a fuse element.

[0012] In view of increased power consumption and high capacity of abattery due to enhanced functions of an electrical appliance, andlegislated product liability, also a thermal fuse is recently requestedto exhibit, for example, aging resistance and heat cycle resistance fora long term, or to have high reliability. In the above-mentionedconventional art examples, In which is a highly reactive element iscontained at a large amount or 50% or more. When the fuse element issubjected particularly to long-term aging, therefore, In in the surfaceof a fuse element reacts with a flux to produce an In salt, and the rateof incorporation into the flux is increased, so that the alloycomposition of the fuse element is changed in the direction of reductionof In. As a result, the variation of the alloy composition shifts theoperating temperature, or increases the resistance of the fuse element,thereby causing reduction of the operating temperature due toself-heating. Furthermore, the function of the flux is reduced, and theoperation characteristic of the thermal fuse is inevitably impaired.Therefore, the long-term aging resistance which is requested in athermal fuse is hardly ensured.

[0013] The aging resistance is requested to be set so that theresistance of a fuse element is not largely changed or a thermal fusedoes not malfunction even when no-load, rated-load, and humidifiedconditions are continued for a long term under an environment of a hightemperature such as the holding temperature (which is the maximumholding temperature where the fuse does not operate even when a ratedcurrent that is obliged to be set by the safety standard is continued tobe supplied for 168 hours, and which is usually set to a temperaturethat is lower than the operating temperature by 20° C.). Theconventional art examples hardly adapt to the long-term agingresistance.

[0014] As a Bi—In—Sn eutectic alloy which can satisfy the requirement ofan operating temperature of 75 to 120° C., and in which the weight of Inis considerably smaller than 50%, there are 79° C.-eutectic (57.5% Bi,25.2% In, and 17.3% Sn) and 81° C.-eutectic (54.0% Bi, 29.7% In, and16.3% Sn). In 79° C.-eutectic, as apparent from FIG. 12 showing a resultof a differential scanning calorimetry analysis [which is called a DSC,and in which a reference specimen (unchanged) and a measurement specimenare housed in an N₂ gas-filled vessel, an electric power is supplied toa heater of the vessel to heat the samples at a constant rate, and avariation of the heat energy input amount due to a state change of themeasurement specimen is detected by a differential thermocouple],however, solid phase transformation occurs in a temperature zone ofabout 52 to 58° C. which is considerably lower than the melting point.In 81° C.-eutectic, as apparent from FIG. 13 showing a result of adifferential scanning calorimetry analysis, solid phase transformationoccurs in a temperature zone of about 51 to 57° C. which is considerablylower than the melting point. As a result of a thermal hysteresisstraddling the transformation temperature zone, a fuse element receivesrepetitive distortion to produce the possibility that the operatingtemperature is lowered by an increased resistance or the fuse element isbroken so as not to operate. Therefore, the long-term heat cyclecharacteristic which is requested in a thermal fuse is hardly ensured.

[0015] The long-term heat cycle characteristic is requested to be set sothat, even when a thermal fuse is subjected to a thermal hysteresisbetween a high temperature (usually, the above-mentioned holdingtemperature) which is lower than the operating temperature and the roomtemperature or a below-freezing temperature (for example, −40° C.), theresistance of a fuse element is not changed or a thermal fuse does notmalfunction. However, the 79° C.- and 81° C.-eutectics hardly adapt tothe long-term heat cycle resistance.

[0016] The melting characteristic of an alloy can be obtained by a DSCmeasurement. The inventor measured and eagerly studied DSCs of Bi—In—Snalloys of various compositions, and found that, depending on thecomposition, the DSCs show melting characteristics of the patterns suchas shown in (A) to (D) of FIG. 14, and, when a Bi—In—Sn alloy of themelt pattern of (A) of FIG. 14 is used as fuse elements, the fuseelements can be concentrically fused off in the vicinity of the maximumendothermic peak.

[0017] The pattern of (A) of FIG. 14 will be described. At the solidustemperature a, an alloy starts to be liquefied (melted). In accordancewith progress of the liquidification, the absorption amount of heatenergy is increased, and reaches the maximum at a peak p. After passingthe point, the absorption amount of heat energy is gradually reduced,and becomes zero at the liquidus temperature b, thereby completing theliquidification. Thereafter, the temperature is raised in the state ofthe liquid phase.

[0018] The reason why a division operation of the fuse element occurs inthe vicinity of the maximum endothermic peak p is estimated as follows.In a BiIn—Sn composition showing such a melting characteristic, allconstituting elements have excellent wettability so as to exhibitexcellent wettability even in the solid-liquid coexisting region in thevicinity of the maximum endothermic peak p in which the liquid phasestate has not yet been completely established. Therefore, spheroiddivision occurs before a state exceeding the solid-liquid coexistingregion is attained.

[0019] In FIG. 14, (B) shows the melt pattern of a eutectic compositionor a composition in the vicinity of the eutectic. In the pattern, thesolid-liquid coexisting region is zero or very narrow.

[0020] In the melt pattern of (C) of FIG. 14 among (C) and (D) of FIG.14, the heat energy is slowly absorbed, and the wettability is notsuddenly changed. Therefore, the point of a division operation of thefuse element is not deter-mined in a narrow range. In the melt patternof (D) of FIG. 14, there are plural endothermic peaks. At any one of theendothermic peaks, a division operation of the fuse element may probablyoccur. In both (C) and (D) of FIG. 14, therefore, the point of adivision operation of the fuse element cannot be concentrated into anarrow range.

[0021] From the result of the above consideration, the followings areeffective for obtaining an environment adaptive alloy type thermal fusein which an excellent operation characteristic can be ensured at anoperating temperature of 75 to 120° C. Because of the unadaptability tothe long-term heat cycle resistance, Bi—In—Sn eutectic alloys of 79°C.-eutectic (57.5% Bi, 25.2% In, and 17.3% Sn), and 81° C.-eutectic(54.0% Bi, 29.7% In, and 16.3% Sn), and those in the range adjacent tothe compositions are excluded. Because of the long-term agingresistance, furthermore, the amount of In is restricted, the operatingtemperature of 75 to 120° C. is satisfied, and the melt pattern fulfillsthat of (A) of FIG. 14 or approaches that of (B) of FIG. 14.

SUMMARY OF THE INVENTION

[0022] It is an object of the invention to, based on the considerationresult, provide an alloy type thermal fuse of an operating temperatureof 75 to 120° C. in which a fuse element of a Bi—In—Sn alloy is used,which exhibits excellent heat cycle and aging resistances for a longterm, and in which satisfactory operating characteristic can be ensured.

[0023] It is a further object of the invention to thin a fuse element toreduce the size and thickness of an alloy type thermal fuse.

[0024] The material for a thermal fuse element of a first aspect of theinvention has an alloy composition in which In is 15% or larger andsmaller than 37%, Sn is 5% or larger and 28% or smaller, and balance Bi,and in which, with respect to each of reference points of ternaryBi—In—Sn eutectic points of 57.5% Bi-25.2% In-17.3% Sn and 54.0%Bi-29.7% In-16.3% Sn, a range of ±2% Bi, ±1% In, and ±1% Sn is excluded.

[0025] In the material for a thermal fuse element of a second aspect ofthe invention, 0.1 to 3.5 weight parts of one, or two or more elementsselected from the group consisting of Ag, Au, Cu, Ni, Pd, Pt, Sb, Ga,and Ge are added to 100 weight parts of the alloy composition of thefirst aspect of the invention.

[0026] The materials for a thermal fuse element are allowed to containinevitable impurities which are produced in productions of metals of rawmaterials and also in melting and stirring of the raw materials, andwhich exist in an amount that does not substantially affect thecharacteristics. In the alloy type thermal fuses, a minute amount of ametal material or a metal film material of the lead conductors or thefilm electrodes is caused to inevitably migrate into the fuse element bysolid phase diffusion, and, when the characteristics are notsubstantially affected, allowed to exist as inevitable impurities.

[0027] In the alloy type thermal fuse of a third aspect of theinvention, the material for a thermal fuse element of the first orsecond aspect of the invention is used as a fuse element.

[0028] The alloy type thermal fuse of a fourth aspect of the inventionis characterized in that, in the alloy type thermal fuse of the thirdaspect of the invention, the fuse element contains inevitableimpurities.

[0029] The alloy type thermal fuse of a fifth aspect of the invention isan alloy type thermal fuse in which, in the alloy type thermal fuse ofthe third or fourth aspect of the invention, the fuse element isconnected between lead conductors, and at least a portion of each of thelead conductors which is bonded to the fuse element is covered with a Snor Ag film.

[0030] The alloy type thermal fuse of a sixth aspect of the invention isan alloy type thermal fuse in which, in the alloy type thermal fuse ofthe third or fourth aspect of the invention, a pair of film electrodesare formed on a substrate by printing conductive paste containing metalparticles and a binder, the fuse element is connected between the filmelectrodes, and the metal particles are made of a material selected fromthe group consisting of Ag, Ag—Pd, Ag—Pt, Au, Ni, and Cu.

[0031] The alloy type thermal fuse of a seventh aspect of the inventionis an alloy type thermal fuse in which, in the alloy type thermal fuseof any one of the third to sixth aspects of the invention, a heatingelement for fusing off the fuse element is additionally disposed.

[0032] The alloy type thermal fuse of an eighth aspect of the inventionis an alloy type thermal fuse in which, in the alloy type thermal fuseof any one of the third to sixth aspects of the invention, the fuseelement connected between a pair of lead conductors is sandwichedbetween insulating films.

[0033] The alloy type thermal fuse of a ninth aspect of the invention isan alloy type thermal fuse in which, in the alloy type thermal fuse ofany one of the third to sixth aspects of the invention, a pair of leadconductors are partly exposed from one face of an insulating plate toanother face, the fuse element is connected to the lead conductorexposed portions, and the other face of the insulating plate is coveredwith an insulating material.

[0034] The alloy type thermal fuse of a tenth aspect of the invention isan alloy type thermal fuse in which, in the alloy type thermal fuse ofany one of the third to fifth aspects of the invention, lead conductorsare bonded to ends of the fuse element, respectively, a flux is appliedto the fuse element, the flux-applied fuse element is passed through acylindrical case, gaps between ends of the cylindrical case and the leadconductors are sealingly closed, ends of the lead conductors have adisk-like shape, and ends of the fuse element are bonded to front facesof the disks.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1 is a view showing an example of the alloy type thermal fuseof the invention;

[0036]FIG. 2 is a view showing another example of the alloy type thermalfuse of the invention;

[0037]FIG. 3 is a view showing a further example of the alloy typethermal fuse of the invention;

[0038]FIG. 4 is a view showing a still further example of the alloy typethermal fuse of the invention;

[0039]FIG. 5 is a view showing a still further example of the alloy typethermal fuse of the invention;

[0040]FIG. 6 is a view showing a still further example of the alloy typethermal fuse of the invention;

[0041]FIG. 7 is a view showing a still further example of the alloy typethermal fuse of the invention;

[0042]FIG. 8 is a view showing an alloy type thermal fuse of thecylindrical case type and its operation state;

[0043]FIG. 9 is a view showing a still further example of the alloy typethermal fuse of the invention;

[0044]FIG. 10 is a view showing a result of a DSC measurement of a fuseelement of Example 1;

[0045]FIG. 11 is a view showing a result of a DSC measurement of a fuseelement of Example 2;

[0046]FIG. 12 is a view showing a result of a DSC measurement of a 79°C. ternary Bi—In—Sn eutectic alloy;

[0047]FIG. 13 is a view showing a result of a DSC measurement of an 81°C. ternary Bi—In—Sn eutectic alloy; and

[0048]FIG. 14 is a view showing various melt patterns of a ternarySn—In—Bi alloy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] In the invention, a fuse element of a circular wire or a flatwire is used. The outer diameter or the thickness is set to 100 to 800μm, preferably, 300 to 600 μm.

[0050] The reasons why, in the first aspect of the invention, a thermalfuse element has an alloy composition in which In is 15% or larger andsmaller than 37%, Sn is 5% or larger and 28% or smaller, and balance Bi,and in which, with respect to each of reference points of 79° C. ternaryBi—In—Sn eutectic point of 57.5% Bi-25.2% In-17.3% Sn and 81° C. ternaryBi—In—Sn eutectic point of 54.0% Bi-29.7% In-16.3% Sn, a range of ±2%Bi, ±1% In, and ±1% Sn is excluded (namely, the range of 55.5%≦Bi≦59.5%,24.2%≦In≦26.2%, and 16.3%≦Sn≦18.3%, and that of 52%<Bi≦56%,28.7%≦In≦30.7%, and 15.3%≦Sn≦17.3% are excluded) are as follows. Inorder to use a Bi—In—Sn alloy because of the adaptability to theenvironment, and satisfy the requirement that the alloy type thermalfuse has an operating temperature of 75 to 120° C., the following pointsare satisfied with respect to the reference points of 79° C.-eutecticand 81° C.-eutectic: (i) the two eutectic points and the ranges adjacentto the eutectic points are excluded in order to eliminate solid phasetransformation appearing in the eutectics; (ii) the amount of In isreduced in order to prevent In which is highly reactive, from reactingwith a flux in the surface of the fuse element to be reduced, andreactive groups of the flux from forming an In salt; and (iii) althoughthe composition shows a melt pattern having a wide solid-liquidcoexisting region which is considerably separated from the eutecticpoints, the alloy composition exhibits a single maximum endothermic peaksuch as shown in (A) of FIG. 14 (according to the alloy composition,namely, the fuse element can operate in a concentrated temperature zone,and dispersion of the operating temperature can be set to be within anallowable range), and the maximum endothermic peak satisfies therequirement of an operating temperature of 75 to 120° C.

[0051] In the above, in interface zones respectively adjacent to theeutectic points in the remaining region excluding the range of ±2% Bi,±1% In, and +1% Sn with respect to each of the 79° C. ternary Bi—In—Sneutectic point and the 81° C. ternary Bi—In—Sn eutectic point, themelting point is close to the melting points of the eutectics (79 to 81°C.), and also the DSC melt pattern is close to the melt patterns of the79° C. ternary Bi—In—Sn eutectic and 81° C. ternary Bi—In—Sn eutectic.Therefore, requirement (iii) is satisfied. In addition, solid phasetransformation in a range which is lower than the melting point can beeliminated, and hence requirement (i) is satisfied. Since the amount ofIn is small, also requirement (ii) is satisfied.

[0052] Each of the aboves will be further described.

[0053] (1) From a result of a DSC measurement of a 79° C. ternaryBi—In—Sn eutectic alloy shown in FIG. 12 and that of a DSC measurementof an 81° C. ternary Bi—In—Sn eutectic alloy shown in FIG. 13, it isseen that the absorption amount of heat energy is sharply changed in thevicinity of the melting point because the solid phase is suddenlychanged to the liquid phase, and, in the temperature zones of about 52to 58° C. and about 51 to 57° C. which are lower than the melting point,the heat energy is absorbed and transformation occurs while maintainingthe solid phase state. In the solid phase transformation, distortion isgenerated in accordance with a change of the phase state, and hencestress is produced in the fuse element ends of which are fixed to leadconductors or electrodes. A thermal fuse is exposed to a heat cycle at atemperature which is lower than the operating temperature. As describedabove, a thermal fuse is requested to have predetermined heat cycleresistance, and to pass a heat cycle test in which one cycle is set tobe between the normal temperature (the operating temperature −20° C.)and the room temperature or a below-freezing temperature (usually, −40°C.). In the case of an operating temperature of 75 to 120° C., one cycleis set to be between (55 to 100° C.) and −40° C., and the solid phasetransformation zones (52 to 58° C.) and (51 to 57° C.) overlap with thecycle. Therefore, stress due to solid phase transformation isrepetitively applied to the fuse element. When this state continues fora long period, a remarkable change of the resistance, breakage, or amalfunction is caused.

[0054] In the invention, therefore, the range of ±2% Bi, ±1% In, and ±1%Sn with respect to each of the 79° C. ternary Bi—In—Sn eutectic pointand the 81° C. ternary Bi—In—Sn eutectic point is excluded.

[0055] (2) In is more highly reactive than Bi and Sn, and reacts in thesurface of a fuse element with reactive groups of the flux to produce anIn salt. When the production rate is high, shift or impairment of themelting characteristic of the fuse element due to the reduced amount ofIn, and reduction of the activity of the flux remarkably occur to impairthe characteristics of the thermal fuse. In a thermal fuse, it isrequested to evaluate the aging resistance, so that abnormality does notoccur even when load, no-load, and humidified conditions are continuedfor a long term under an environment of a high temperature such as theholding temperature. Because of the impairment of the characteristics ofthe thermal fuse due to the reaction of In, however, it is verydifficult to maintain the operation stability for a long period.

[0056] In the invention, therefore, the amount of In is set to besmaller than that in Patent literatures 1 to 6 above or to be smallerthan 37%. In this case, since the range of In smaller than 15% isexcluded, the requirement of an operating temperature of 75 to 120° C.is satisfied, and thinning to 300 μmφ can be performed with a highyield.

[0057] (3) In Bi—In—Sn alloys, there is an alloy having a melt patternin which, even when deviated from a eutectic point or a eutectic line,or when the solid-liquid coexisting region is widened, the maximumendothermic peak is at one point in the wide solid-liquid coexistingregion as shown in (A) of FIG. 14. In such an alloy, in the endothermicbehavior in the melting process, the heat absorption amount differenceat the maximum endothermic peak is very larger than that in anotherportion of the endothermic process, and all constituting elements haveexcellent wettability. Therefore, the wettability of the solid-liquidcoexisting region at the maximum endothermic peak is sufficientlyimproved even before the completion of the liquidification, so thatspheroid division of the thermal fuse element can be performed in thevicinity of the maximum endothermic peak.

[0058] In the invention, therefore, Sn is set to 5 to 28% so that,although deviated from the 79° C. ternary Bi—In—Sn eutectic point andthe 81° C. ternary Bi—In—Sn eutectic point, the operating temperature isset to the range of 75 to 120° C. with dispersion of an allowable range(±5° C.).

[0059] In the first aspect of the invention, one of the reference alloycompositions is that In is 25%, Sn is 20%, and a balance is Bi. Theliquidus temperature is about 84° C., the solidus temperature is about80° C., a result of a DSC measurement at a temperature rise rate of 5°C./min. is shown in FIG. 10, and the maximum endothermic peak is atabout 82° C.

[0060] The other reference composition is that In is 30%, Sn is 15%, anda balance is Bi. The liquidus temperature is about 86° C., the solidustemperature is about 81° C., a result of a DSC measurement at atemperature rise rate of 5° C./min. is shown in FIG. 11, and the maximumendothermic peak is at about 82° C.

[0061] In both the measurement results, an endothermic reaction is notobserved in a temperature region which is lower than the melting pointsobserved in the DSC measurement result of the 79° C. ternary Bi—In—Sneutectic alloy shown in FIG. 12 and that of the 81° C. ternary Bi—In—Sneutectic alloy shown in FIG. 13, and there is no solid phasetransformation which may cause a serious problem.

[0062] In the invention, 0.1 to 3.5 weight parts of one, or two or moreelements selected from the group consisting of Ag, Au, Cu, Ni, Pd, Pt,Sb, Ga, and Ge are added to 100 weight parts of the alloy composition,in order to reduce the specific resistance of the alloy and improve themechanical strength. When the addition amount is smaller than 0.1 weightparts, the effects cannot be sufficiently attained, and, when theaddition amount is larger than 3.5 weight parts, the above-mentionedmelting characteristic is hardly maintained.

[0063] With respect to a drawing process, further enhanced strength andductility are provided so that drawing into a thin wire of 100 to 300μmφ can be easily conducted. In the case where the cohesive force of afuse element alloy is considerably enhanced by the inclusion of In, evenwhen a fuse element is insufficiently welded or bonded to leadconductors or the like, a superficial appearance in which the element isbonded is produced. The addition of the element(s) can reduce thecohesive force, so that this defect can be eliminated, and the accuracyof the acceptance criterion in a test after welding can be improved.

[0064] It is known that a to-be-bonded material such as a metal materialof the lead conductors, a thin-film material, or a particulate metalmaterial in the film electrode migrates into the fuse element by solidphase diffusion. When the same element as the to-be-bonded material,such as Ag, Au, Cu, or Ni is previously added to the fuse element, themigration can be suppressed. Therefore, an influence of the to-be-bondedmaterial which may originally affect the characteristics (for example,Ag, Au, or the like causes local reduction or dispersion of theoperating temperature due to the lowered melting point, and Cu, Ni, orthe like causes dispersion of the operating temperature or an operationfailure due to an increased intermetallic compound layer formed in theinterface between different phases) is eliminated, and the thermal fusecan be assured to normally operate, without impairing the function ofthe fuse element.

[0065] The fuse element of the alloy type thermal fuse of the inventioncan be usually produced by a method in which a billet is produced, thebillet is shaped into a stock wire by an extruder, and the stock wire isdrawn by a dice to a wire. The outer diameter is 100 to 800 μmφ,preferably, 300 to 600 μm+. The wire can be finally passed throughcalender rolls so as to be used as a flat wire.

[0066] Alternatively, the fuse element may be produced by the rotarydrum spinning method in which a cylinder containing cooling liquid isrotated, the cooling liquid is held in a layer-like manner by arotational centrifugal force, and a molten material jet ejected from anozzle is introduced into the cooling liquid layer to be cooled andsolidified, thereby obtaining a thin wire member.

[0067] In the production, the alloy composition is allowed to containinevitable impurities which are produced in productions of metals of rawmaterials and also in melting and stirring of the raw materials.

[0068] The invention may be implemented in the form of a thermal fuseserving as an independent thermoprotector. Alternatively, the inventionmay be implemented in the form in which a thermal fuse element isconnected in series to a semiconductor device, a capacitor, or aresistor, a flux is applied to the element, the flux-applied fuseelement is placed in the vicinity of the semiconductor device, thecapacitor, or the resistor, and the fuse element is sealed together withthe semiconductor device, the capacitor, or the resistor by means ofresin mold, a case, or the like.

[0069] The thermal fuse of the invention is useful particularly as athermoprotector for a secondary battery of a high energy density such asa lithium battery or a lithium polymer battery, and configuredpreferably as a thin thermal fuse of the tape type in view of theaccommodation space in a battery pack.

[0070]FIG. 1 is a view showing an embodiment of a thin thermal fuse.

[0071] Referring to FIG. 1, 1 denotes flat lead conductors, and 2denotes a fuse element of the first or second aspect of the inventionwhich is bonded between upper faces of tip ends of the flat leadconductors 1 by welding or the like. In the welding process, spotresistance welding, laser welding, or the like can be used. Thereference numeral 41 denotes a lower resin film, and 42 denotes an upperresin film. Front end portions of the flat lead conductors 1, and thefuse element 2 are sandwiched between the resin films 41, 42, and theperipheral portion of the upper resin film 42 is sealingly bonded to thelower resin film 41 which is horizontally held. The reference numeral 3denotes a flux applied to the periphery of the fuse element 2.

[0072] The thin thermal fuse is produced in the following manner. Thefuse element is bonded between the upper faces of the tip ends of theflat lead conductors by spot resistance welding, laser welding, or thelike. Front end portions of the flat lead conductors 1, and the fuseelement 2 are sandwiched between the lower and upper resin films 41, 42,the lower resin film 41 is horizontally held on a platform, and endportions of the upper resin film 42 are pressed by a releasable chipsuch as a ceramic chip to cause end portions 421 of the upper resin film42 to be in press contact with the flat lead conductors 1. Under thisstate, the flat lead conductors 1 are heated so that the contact facesof the flat lead conductors 1 and end portions (portions pressed by thereleasable chip) of the resin films 41, 42 are fusingly bonded together.Thereafter, faces of the resin films 41, 42 which are directly incontact with each other are sealingly bonded together. The timing ofapplying the flux 3 is set to that before the fuse element 2 issandwiched between the lower and upper resin films 41, 42, or that afterthe contact faces of the flat lead conductors 1 and end portions of theresin films 41, 42 are fusingly bonded together and before faces of theresin films 41, 42 which are directly in contact with each other aresealingly bonded together.

[0073] The flat lead conductors can be heated by electromagneticinduction heating, contact between a heat plate and the lead conductors,or the like. In electromagnetic induction heating, particularly,high-frequency magnetic fluxes cross tip end portions of the leadconductors welded to end portions of the fuse element, through the loweror upper resin film to concentrically heat the tip end portions.Therefore, electromagnetic induction heating is advantageous from theviewpoint of the heat efficiency. The seal bonding between the faces ofthe lower and upper resin films 41, 42 which are directly in contactwith each other can be performed by ultrasonic fusion, high-frequencyinduction heating fusion, heat plate contact fusion, or the like.

[0074]FIG. 2 is a view showing another embodiment of a thin thermalfuse.

[0075] Referring to FIG. 2, 41 denotes a resin base film, and 1 denotesflat lead conductors in each of which a front end portion is fixed tothe rear face of the base film 41 and a part 10 of the front portion isexposed from the upper face of the base film 41. The reference numeral 2denotes a fuse element of the first or second aspect of the inventionwhich is bonded between the exposed portions 10 of the flat leadconductors 1 by welding or the like. In the welding process, spotresistance welding, laser welding, or the like can be used. Thereference numeral 42 denotes a resin cover film which is sealinglybonded in a peripheral portion to the base film 41 that is horizontallyheld. The reference numeral 3 denotes a flux applied to the periphery ofthe fuse element 2.

[0076] The exposure of the portions 10 of the flat lead conductors 1 maybe conducted by, for example, one of the following methods. A projectionis previously formed in the front end portion of each of the flat leadconductors by a squeezing process, the front end portions of the flatlead conductors are fusingly bonded under heating to the rear face ofthe base film, and the projections are protrudingly bonded to the basefilm. Alternatively, the front end portions of the flat lead conductorsare fusingly bonded under heating to the rear face of the base film, andparts of the front end portions of the flat lead conductors are causedto appear from the surface of the base film by a squeezing process.

[0077] The thin thermal fuse is produced in the following manner. On aplatform, the fuse element 2 is bonded between the lead conductorexposed portions 10 of the surface of the resin base film 41 by spotresistance welding, laser welding, or the like. The flux 3 is thenapplied to the fuse element 2. Thereafter, the resin cover film 42 isplaced, and the peripheral portion of the film is sealingly bonded tothe periphery of the resin base film 41.

[0078] The seal bonding of the peripheral portion of the resin coverfilm 42 to the resin base film 41 can be performed by ultrasonic fusion,high-frequency induction heating fusion, heat plate contact fusion, orthe like.

[0079] The thermal fuse of the invention may be realized in the form ofa fuse of the case type, the substrate type, or the like.

[0080]FIG. 3 shows an alloy type thermal fuse of the cylindrical casetype according to the invention. A fuse element 2 of the first or secondaspect of the invention is connected between a pair of lead conductors 1by, for example, welding. A flux 3 is applied to the fuse element 2. Theflux-applied fuse element is passed through an insulating tube 4 whichis excellent in heat resistance and thermal conductivity, for example, aceramic tube. Gaps between the ends of the insulating tube 4 and thelead conductors 1 are sealingly closed by a sealing agent 5 such as acold-setting epoxy resin.

[0081]FIG. 4 shows a fuse of the radial case type. A fuse element 2 ofthe first or second aspect of the invention is connected between tipends of parallel lead conductors 1 by, for example, welding. A flux 3 isapplied to the fuse element 2. The flux-applied fuse element is enclosedby an insulating case 4 in which one end is opened, for example, aceramic case. The opening of the insulating case 4 is sealingly closedby sealing agent 5 such as a cold-setting epoxy resin.

[0082]FIG. 5 shows a fuse of the radial resin dipping type. A fuseelement 2 of the first or second aspect of the invention is bondedbetween tip ends of parallel lead conductors 1 by, for example, welding.A flux 3 is applied to the fuse element 2. The flux-applied fuse elementis dipped into a resin solution to seal the element by an insulativesealing agent such as an epoxy resin 5.

[0083]FIG. 6 shows a fuse of the substrate type. A pair of filmelectrodes 1 are formed on an insulating substrate 4 such as a ceramicsubstrate by printing conductive paste. Lead conductors 11 are connectedrespectively to the electrodes 1 by, for example, welding or soldering.A fuse element 2 of the first or second aspect of the invention isbonded between the electrodes 1 by, for example, welding. A flux 3 isapplied to the fuse element 2. The flux-applied fuse element is coveredwith a sealing agent 5 such as an epoxy resin. The conductive pastecontains metal particles and a binder. For example, Ag, Ag-—d, Ag—Pt,Au, Ni, or Cu may be used as the metal particles, and a materialcontaining a glass frit, a thermosetting resin, and the like may be usedas the binder.

[0084] The invention may be implemented in the form in which a heatingelement for fusing off the fuse element is additionally disposed on thealloy type thermal fuse. As shown in FIG. 7, for example, a conductorpattern 100 having fuse element electrodes 1 and resistor electrodes 10is formed on an insulating substrate 4 such as a ceramic substrate byprinting conductive paste, and a film resistor 6 is disposed between theresistor electrodes 10 by applying and baking resistance paste (e.g.,paste of metal oxide powder such as ruthenium oxide). Lead conductors 11are bonded respectively to the electrodes 1 and 10. A fuse element 2 ofthe first or second aspect of the invention is bonded between the fuseelement electrodes 1 by, for example, welding. A flux 3 is applied tothe fuse element 2. The flux-applied fuse element 2 and the filmresistor 6 are covered with a sealing agent 5 such as an epoxy resin. Inthe thermal fuse having an electric heating element, a precursor causingabnormal heat generation of an appliance is detected, the film resistoris energized to generate heat in response to a signal indicative of thedetection, and the fuse element is fused off by the heat generation.

[0085] The heating element may be disposed on the upper face of aninsulating substrate. A heat-resistant and thermal-conductive insulatingfilm such as a glass baked film is formed on the heating element. A pairof electrodes are disposed, flat lead conductors are connectedrespectively to the electrodes, and the fuse element is connectedbetween the electrodes. A flux covers a range over the fuse element andthe tip ends of the lead conductors. An insulating cover is placed onthe insulating substrate, and the periphery of the insulating cover issealingly bonded to the insulating substrate by an adhesive agent.

[0086] Among the alloy type thermal fuses, those of the type in whichthe fuse element is directly bonded to the lead conductors (FIGS. 1 to5) may be configured in the following manner. At least portions of thelead conductors where the fuse element is bonded are covered with a thinfilm of Sn or Ag (having a thickness of, for example, 15 μm or smaller,preferably, 5 to 10 μm) (by plating or the like), thereby enhancing thebonding strength with respect to the fuse element.

[0087] In the alloy type thermal fuses, there is a possibility that ametal material or a thin film material in the lead conductors, or aparticulate metal material in the film electrode migrates into the fuseelement by solid phase diffusion. As described above, however, thecharacteristics of the fuse element can be sufficiently maintained bypreviously adding the same element as the thin film material into thefuse element.

[0088] As the flux, a flux having a melting point which is lower thanthat of the fuse element is generally used. For example, useful is aflux containing 90 to 60 weight parts of rosin, 10 to 40 weight parts ofstearic acid, and 0 to 3 weight parts of an activating agent. In thiscase, as the rosin, a natural rosin, a modified rosin (for example, ahydrogenated rosin, an inhomogeneous rosin, or a polymerized rosin), ora purified rosin thereof can be used. As the activating agent,hydrochloride or hydrobromide of an amine such as diethylamine, or anorganic acid such as adipic acid can be used.

[0089] As the resin film of the thin thermal fuse, useful is a plasticfilm having a thickness of about 100 to 500 μm, for example, a film of:an engineering plastic such as polyethylene terephtalate, polyethylenenaphthalate, polyamide, polyimide, polybuthylene terephtalate,polyphenylene oxide, polyethylene sulfide, or polysulfone; anengineering plastic such as polyacetal, polycaronate, polyphenylenesulfide, polyoxybenzoyl, polyether ether ketone, or polyether imide;polypropylene; polyvinyl chloride; polyvinyl acetate; polymetylmethacrylate; polyvinylidene chloride; polytetrafluoroethylene; ethylenepolytetrafluoroethylene copolymer; ethylene-vinyl acetate copolymer(EVA); AS resin; ABS resin; ionomer; AAS resin; or ACS resin.

[0090] Among the above-described alloy type thermal fuses, in the fuseof the cylindrical case type, the arrangement in which the leadconductors 1 are placed so as not to be eccentric to the cylindricalcase 4 as shown in (A) of FIG. 8 is a precondition to enable the normalspheroid division shown in (B) of FIG. 8. When the lead conductors areeccentric as shown in (C) of FIG. 8, the flux (including a charred flux)and scattered alloy portions easily adhere to the inner wall of thecylindrical case after an operation as shown in (D) of FIG. 8. As aresult, the insulation resistance is lowered, and the dielectricbreakdown characteristic is impaired.

[0091] In order to prevent such disadvantages from being produced, asshown in (A) of FIG. 9, a configuration is effective in which ends ofthe lead conductors 1 are formed into a disk-like shape d, and ends ofthe fuse element 2 are bonded to the front faces of the disks d,respectively (by, for example, welding). The outer peripheries of thedisks are supported by the inner face of the cylindrical case, and thefuse element 2 is positioned so as to be substantially concentrical withthe cylindrical case 4 [in (A) of FIG. 9, 3 denotes a flux applied tothe fuse element 2, 4 denotes the cylindrical case, 5 denotes a sealingagent such as an epoxy resin, and the outer diameter of each disk isapproximately equal to the inner diameter of the cylindrical case]. Inthis instance, as shown in (B) of FIG. 9, molten portions of the fuseelement spherically aggregate on the front faces of the disks d, therebypreventing the flux (including a charred flux) from adhering to theinner face of the case 4.

EXAMPLES

[0092] In the following examples and comparative examples, alloy typethermal fuses of the thin type shown in FIG. 1 were used. Apolybuthylene terephtalate film having a thickness of 200 μm, a width of5 mm, and a length of 10 mm was used as the lower resin film 41 and theupper resin film 42. A copper conductor having a thickness of 150 μm, awidth of 3 mm, and a length of 20 mm was used as the flat leadconductors 1. The fuse element 2 has a length of 4 mm and an outerdiameter of 300 μmφ. A compound of 80 weight parts of natural rosin, 20weight parts of stearic acid, and 1 weight part of hydrobromide ofdiethylamine was used as the flux.

[0093] The solidus and liquidus temperatures of a fuse element weremeasured by a DSC at a temperature rise rate of 5° C./min.

[0094] Fifty specimens were used. Each of the specimens was immersedinto an oil bath in which the temperature was raised at a rate of 1°C./min., while supplying a current of 0.1 A to the specimen, and thetemperature T0 of the oil when the current supply was interrupted byblowing-out of the fuse element was measured. A temperature of T0-2° C.was determined as the element temperature at an operation of the thermalfuse.

[0095] The heat cycle resistance was evaluated in the following manner.Fifty specimens were used. A heat cycle test in which each cycle isconfigured by (operating temperature −20° C.)×30 min. and −40° C.×30min. was conducted 1,000 cycles. The resistance was measured. When anabnormality such as that the resistance is changed remarkably or by 50%or more, that the fuse element is broken, or that, in an after-testoperation test, the operating temperature is deviated by ±7° C. or morefrom the initial operating temperature or the thermal fuse does notoperate was observed even in one specimen, the heat cycle resistance wasevaluated as unacceptable. When an abnormality was not observed in allthe specimens, the heat cycle resistance was evaluated as acceptable.

[0096] The aging resistance was evaluated by a load aging test. Fiftyspecimens were used. The specimens were exposed to a high-temperatureenvironment of (operating temperature −20° C.) for 20,000 hours whilesupplying a rated current. Thereafter, the resistance was measured. Whenan abnormality such as that the resistance is changed remarkably or by50% or more, that the fuse element is broken, or that, in an after-testoperation test, the operating temperature is deviated by +7° C. or morefrom the initial operating temperature or the thermal fuse does notoperate was observed even in one specimen, the aging resistance wasevaluated as unacceptable. When an abnormality was not observed in allthe specimens, the aging resistance was evaluated as acceptable.

[0097] With respect to the drawability of a fuse element, a process ofdrawing to 300 μmφ under the conditions of an area reduction per dice of6.5%, and a drawing speed of 50 m/min. was conducted. When the drawingprocess was conducted with satisfactory yield without causing aconstricted portion or a breakage, the drawability was evaluated as ◯.When a constricted portion or a breakage was caused so that thesectional area was not stabilized nor the continuity of the drawing wasnot ensured, the drawability was evaluated as x.

Example 1

[0098] A fuse element having an alloy composition of 25% In, 20% Sn, andbalance Bi was produced. The wire drawability to a fuse element was ◯.

[0099]FIG. 10 shows a result of a DSC measurement of the fuse element.The liquidus temperature was about 84° C., the solidus temperature wasabout 80° C., and the maximum endothermic peak temperature was about 81°C. Since the alloy composition is close to the 79° C. ternary Bi—In—Sneutectic point of 57.5% Bi-25.2% In-17.3% Sn, the DSC measurement resultbelongs to the pattern of (B) FIG. 14. However, the solid phasetransformation zone does not exist in the temperature side which islower than the solidus temperature.

[0100] The fuse element temperature at an operation of a thermal fusewas 82±1° C. Therefore, it is apparent that the fuse element temperatureat an operation of a thermal fuse approximately coincides with themaximum endothermic peak temperature of about 82° C.

[0101] The example passed both the load aging test and the heat cycletest. The reason of the pass in the load aging test is estimated asfollows. Since the amount of In is as small as 25%, the reaction of Inwith the flux was suppressed, and the variation of the alloy compositionand the reduction of the activity of the flux were conducted at a verysmall degree. As apparent from the DSC measurement result, solid phasetransformation was not observed in the temperature side which is lowerthan the solidus temperature. Therefore, the pass in the heat cycle testcoincides with the estimation.

Example 2

[0102] A fuse element having an alloy composition of 30% In, 15% Sn, andbalance Bi was produced.

[0103] The wire drawability to a fuse element was ◯.

[0104]FIG. 11 shows a result of a DSC measurement of the fuse element.The liquidus temperature was about 86° C., the solidus temperature wasabout 79° C., and the maximum endothermic peak temperature was about 82°C. Since the alloy composition is close to the 81° C. ternary Bi—In—Sneutectic point of 54.0% Bi-29.7% In-16.3% Sn, the DSC measurement resultbelongs to the pattern of (B) FIG. 14. However, the solid phasetransformation zone does not exist in the temperature side which islower than the solidus temperature.

[0105] The fuse element temperature at an operation of a thermal fusewas 82±1° C. Therefore, it is apparent that the fuse element temperatureat an operation of a thermal fuse approximately coincides with themaximum endothermic peak temperature of about 82° C.

[0106] The example passed both the load aging test and the heat cycletest. The reason of the pass in the load aging test is estimated asfollows. Since the amount of In is as small as 30%, the reaction of Inwith the flux was suppressed, and the variation of the alloy compositionand the reduction of the activity of the flux were conducted at a verysmall degree in the same manner as Example 1. As apparent from the DSCmeasurement result, in the same manner as Example 1, solid phasetransformation was not observed in the temperature side which is lowerthan the solidus temperature. Therefore, the pass in the heat cycle testcoincides with the estimation.

Examples 3 to 7

[0107] The examples were conducted in the same manner as Example 1except that the alloy composition in Example 1 was changed as listed inTable 1.

[0108] In all the examples, good wire drawability was obtained.

[0109] The solidus and liquidus temperatures of the examples are shownin Table 1. The fuse element temperatures at an operation are as shownin Table 1, have dispersion of ±3° C. or smaller, and are in thesolid-liquid coexisting region.

[0110] The melt pattern of the fuse element of each example belongs tothe pattern of (A) of FIG. 14, and the solid-liquid coexisting region iswide. However, the single endothermic peak exists and is sharp. As aresult, dispersion of the operating temperature can be set to be ±3° C.or smaller.

[0111] The examples passed the load aging test. The reason of the passin the load aging test is estimated as follows. Since the amount of Inis as small as 15 to 30%, the reaction of In with the flux wassuppressed, and the variation of the alloy composition and the reductionof the activity of the flux were conducted at a very small degree in thesame manner as Example 1.

[0112] The examples passed also the heat cycle test. From results of DSCmeasurements, it was confirmed that solid phase transformation does notexist in the temperature side which is lower than the solidustemperature. This coincides with the estimation.

[0113] [Table 1] TABLE 1 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 In (%) 15 20 2530 35 Sn (%)  5  5  5  5  5 Bi Balance Balance Balance Balance BalanceSolidus temperature 79 79 79 80 84 (° C.) Liquidus temperature 194  171 144  109  105  (° C.) Element temperature 85 ± 1 84 ± 1 92 ± 2 95 ± 3 98± 3 at operation (° C.) Heat cycle resistance Passed Passed PassedPassed Passed test Load aging test Passed Passed Passed Passed Passed

Examples 8 to 11

[0114] The examples were conducted in the same manner as Example 1except that the alloy composition in Example 1 was changed as listed inTable 2.

[0115] In all the examples, good wire drawability was obtained.

[0116] The solidus and liquidus temperatures of the examples are shownin Table 2. The fuse element temperatures at an operation are as shownin Table 2, have dispersion of ±1° C. or smaller, and are in thesolid-liquid coexisting region.

[0117] The melt pattern of the fuse element of each example belongs tothe pattern of (A) of FIG. 14, and the solid-liquid coexisting region iswide. However, the single endothermic peak exists and is sharp. As aresult, dispersion of the operating temperature can be set to be ±1° C.or smaller.

[0118] The examples passed the load aging test. The reason of the passin the load aging test is estimated as follows. Since the amount of Inis as small as 15 to 35%, the reaction of In with the flux wassuppressed, and the variation of the alloy composition and the reductionof the activity of the flux were conducted at a very small degree in thesame manner as Example 1.

[0119] The examples passed also the heat cycle test. From results of DSCmeasurements, it was confirmed that solid phase transformation does notexist in the temperature side which is lower than the solidustemperature. This coincides with the estimation.

[0120] [Table 2] TABLE 2 Ex. 8 Ex. 9 Ex. 10 Ex. 11 In (%) 15 20 25 35 Sn(%) 15 15 15 15 Bi Balance Balance Balance Balance Solidus temperature(° C.) 79 80 80 69 Liquidus temperature (° C.) 158  134  105  84 Wiredrawability ◯ ◯ ◯ ◯ Element 86 ± 1 86 ± 1 83 ± 1 79 ± 1 temperature atoperation (° C.) Heat cycle resistance test Passed Passed Passed PassedLoad aging test Passed Passed Passed Passed

Examples 12 to 16

[0121] The examples were conducted in the same manner as Example 1except that the alloy composition in Example 1 was changed as listed inTable 3.

[0122] In all the examples, good wire drawability was obtained.

[0123] The solidus and liquidus temperatures of the examples are shownin Table 3. The fuse element temperatures at an operation are as shownin Table 3, have dispersion of +3° C. or smaller, and are in thesolid-liquid coexisting region.

[0124] The melt pattern of the fuse element of each example belongs tothe pattern of (A) of FIG. 14, and the solid-liquid coexisting region iswide. However, the single endothermic peak exists and is sharp. As aresult, dispersion of the operating temperature can be set to be ±3° C.or smaller.

[0125] The examples passed the load aging test. The reason of the passin the load aging test is estimated as follows. Since the amount of Inis as small as 15 to 35%, the reaction of In with the flux wassuppressed, and the variation of the alloy composition and the reductionof the activity of the flux were conducted at a very small degree in thesame manner as Example 1.

[0126] The examples passed also the heat cycle test. From results of DSCmeasurements, it was confirmed that solid phase transformation does notexist in the temperature side which is lower than the solidustemperature. This coincides with the estimation.

[0127] [Table 3] TABLE 3 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Ex. 16 In (%) 15 2025 30 35 Sn (%) 25 25 25 25 25 Bi Balance Balance Balance BalanceBalance Solidus temperature 79 79 79 78 77 (° C.) Liquidus temperature126  107  107  107  104  (° C.) Wire drawability ◯ ◯ ◯ ◯ ◯ Elementtemperature 94 ± 3 83 ± 1 82 ± 1 81 ± 1 80 ± 3 at operation (° C.) Heatcycle resistance Passed Passed Passed Passed Passed test Load aging testPassed Passed Passed Passed Passed

Example 17

[0128] The example was conducted in the same manner as Example 1 exceptthat an alloy composition in which 1 weight part of Ag was added to 100weight parts of the alloy composition of Example 1 was used as that of afuse element.

[0129] A wire member for a fuse element of 300 μm was produced underconditions in which the area reduction per dice was 8% and the drawingspeed was 80 m/min., and which are severer than those of the drawingprocess of a wire member for a fuse element in Example 1. However, nowire breakage occurred, and problems such as a constricted portion werenot caused, with the result that the example exhibited excellentworkability.

[0130] The solidus temperature was 79° C., and the maximum endothermicpeak temperature and the fuse element temperature at an operation of athermal fuse were lowered only by about 1° C. as compared with those inExample 1. Namely, it was confirmed that the operating temperature andthe melting characteristic can be held without being largelydifferentiated from those of Example 1.

[0131] The example passed both the heat cycle test and the load agingtest. It is estimated that the consideration results were maintainedbecause the addition amount of Ag is as small as 1 weight part.

[0132] It was confirmed that the above-mentioned effects are obtained inthe range of the addition amount of 0.1 to 3.5 weight parts of Ag.

[0133] In the case where the metal material of the lead conductors to bebonded, a thin film material, or a particulate metal material in thefilm electrode is Ag, it was confirmed that, when the same element or Agis previously added as in the example, the metal material can beprevented from, after a fuse element is bonded, migrating into the fuseelement with time by solid phase diffusion, and local reduction ordispersion of the operating temperature due to solid phase diffusion canbe eliminated.

Examples 18 to 25

[0134] The examples were conducted in the same manner as Example 1except that an alloy composition in which 0.5 weight parts of respectiveone of Au, Cu, Ni, Pd, Pt, Ga, Ge, and Sb were added to 100 weight partsof the alloy composition of Example 1 was used as that of a fuseelement.

[0135] It was confirmed that, in the same manner as the metal additionof Ag in Example 17, also the addition of Au, Cu, Ni, Pd, Pt, Ga, Ge, orSb realizes excellent wire drawability, the operating temperature andmelting characteristic are not largely different from those of Example1, the examples passed the heat cycle test and the load aging test, andsolid phase diffusion between metal materials of the same kind can besuppressed.

[0136] It was confirmed that the above-mentioned effects are obtained inthe range of the addition amount of 0.1 to 3.5 weight parts ofrespective one of Au, Cu, Ni; Pd, Pt, Ga, Ge, and Sb.

[0137] [Comparative Example 1]

[0138] The comparative example was conducted in the same manner asExample 1 except that the composition of the fuse element in Example 1was changed to 25.2% In, 17.3% Sn, and the balance Bi.

[0139] The wire drawability was satisfactory. The fuse elementtemperature at an operation of a thermal fuse was 81±1° C. FIG. 12 showsa result of a DSC measurement. It was expected to produce excellentthermal fuses in which the solid-liquid coexisting region is narrow andthe operating temperature is less dispersed. However, solid phasetransformation was observed between temperatures of 52 to 58° C.

[0140] The resistances of specimens which were subjected to 1,000 cyclesof a heat cycle test (in which each cycle is configured by 60° C.×30min. and −40° C.×30 min.) were measured. As a result, a resistancechange of 50% or more, and a breakage often occurred, and the result ofthe heat cycle test was x. This was caused by the following reason. Thesolid phase transformation zone overlaps with the temperature zone ofthe heat cycles, and stress due to solid phase transformation wasrepetitively produced.

[0141] [Comparative Example 2]

[0142] The comparative example was conducted in the same manner asExample 1 except that the composition of the fuse element in Example 1was changed to 29.7% In, 16.3% Sn, and the balance Bi.

[0143] The wire drawability was satisfactory. The fuse elementtemperature at an operation of a thermal fuse was 81+1° C. FIG. 13 showsa result of a DSC measurement. It was expected to produce excellentthermal fuses in which the solid-liquid coexisting region is narrow andthe operating temperature is less dispersed. However, solid phasetransformation was observed between temperatures of 51 to 57° C.

[0144] The resistances of specimens which were subjected to 1,000 cyclesof a heat cycle test (in which each cycle is configured by 60° C.×30min. and −40° C.×30 min.) were measured. As a result, in the same manneras Comparative Example 1, a resistance change of 50% or more, and abreakage often occurred, and the result of the heat cycle test was x.This was caused by the following reason. In the same manner asComparative Example 1, the solid phase transformation zone overlaps withthe temperature zone of the heat cycles, and stress due to solid phasetransformation was repetitively produced.

[0145] [Comparative Example 3]

[0146] The comparative example was conducted in the same manner asExample 1 except that the composition of the fuse element in Example 1was changed to 40% In, 20% Sn, and the balance Bi.

[0147] The wire drawability was satisfactory. As a result of a DSCmeasurement, the solid-liquid coexisting region is narrow. As a resultof the measurement of an operating temperature, dispersion of theoperating temperature was within the allowable range. The result of aheat cycle test was acceptable.

[0148] The resistances of specimens which had been subjected to a loadaging test for 7,000 hours were measured. A remarkable increase of theresistance which is 50% or more was observed. The operating temperaturewas measured. As a result, in many specimens, the operating temperaturewas largely deviated from the range of the initial operating temperature±7° C. The reasons of the above are estimated as follows. In wasconsumed by the flux, and the specific resistance of the fuse elementwas increased. Since the amount of In in the alloy was reduced, theoperating temperature was varied. Since the reactive groups were usedfor producing an In salt, the activity of the flux was reduced, so thatspheroid division of the molten alloy was not satisfactorily conducted.

[0149] [Comparative Example 4]

[0150] The comparative example was conducted in the same manner asExample 1 except that the composition of the fuse element in Example 1was changed to 10% In, 20% Sn, and the balance Bi.

[0151] A process of drawing to 300 μmφ was attempted. However, breakagefrequently occurred, and the wire drawability was x.

[0152] A thin wire of 300 μmφ was obtained by the rotary drum spinningmethod to be formed as a fuse element.

[0153] The DSC measurement result of the fuse element belongs to themelt pattern of (C) of FIG. 14. The fuse element temperature at anoperation was measured. As a result, dispersion was larger than theallowable range of +5° C., and the fuse element was not able to be usedas a thermal fuse.

[0154] The reasons of the large dispersion of the operating temperatureare estimated as follows. The heat energy is slowly absorbed. Thewettability is not suddenly changed. The point of a division operationof the fuse element is not determined in a narrow range.

[0155] [Comparative Example 5]

[0156] The comparative example was conducted in the same manner asExample 1 except that the composition of the fuse element in Example 1was changed to 20% In, 35% Sn, and the balance Bi.

[0157] A drawing process was smoothly conducted, and the wiredrawability was ◯.

[0158] In the result of a DSC measurement, the solid-liquid coexistingregion is wide, the heat energy is slowly absorbed in the solid-liquidcoexisting region, and the wettability is not suddenly changed. The DSCmeasurement result belongs to the melt pattern of (C) of FIG. 14.

[0159] The fuse element temperature at an operation was measured. As aresult, dispersion was larger than the allowable range of ±5° C., andthe fuse element was not able to be used as a thermal fuse.

[0160] The reason of the large dispersion of the operating temperatureis identical with that of Comparative Example 4.

[0161] [Comparative Example 6]

[0162] The comparative example was conducted in the same manner asExample 1 except that the composition of the fuse element in Example 1was changed to 52% In and the balance Bi.

[0163] The wire drawability was satisfactory. As a result of a DSCmeasurement, the solid-liquid coexisting region is narrow. As a resultof the measurement of an operating temperature, dispersion of theoperating temperature was very small. The result of a heat cycle testwas acceptable.

[0164] The resistances of specimens which had been subjected to a loadaging test for 7,000 hours were measured. A remarkable increase of theresistance which is 50% or more was observed. The operating temperaturewas measured. As a result, in many specimens, the operating temperaturewas largely deviated from the range of the initial operating temperature±7° C. The reasons of the above are estimated as follows. In wasconsumed by the flux, and the specific resistance of the fuse elementwas increased. Since the amount of In in the alloy was reduced, theoperating temperature was varied. Since the reactive groups were usedfor producing an In salt, the activity of the flux was reduced, so thatspheroid division of the molten alloy was not satisfactorily conducted.

[0165] [Comparative Example 7]

[0166] The comparative example was conducted in the same manner asExample 1 except that the composition of the fuse element in Example 1was changed to 52% In and the balance Sn.

[0167] The wire drawability was satisfactory. As a result of a DSCmeasurement, the solid-liquid coexisting region is narrow. As a resultof the measurement of an operating temperature, dispersion of theoperating temperature was very small. The result of a heat cycle testwas acceptable.

[0168] The resistances of specimens which had been subjected to a loadaging test for 7,000 hours were measured. A remarkable increase of theresistance which is 50% or more was observed. The operating temperaturewas measured. As a result, in many specimens, the operating temperaturewas largely deviated from the range of the initial operating temperature±7° C. The reasons of the above are estimated as follows. In wasconsumed by the flux, and the specific resistance of the fuse elementwas increased. Since the amount of In in the alloy was reduced, theoperating temperature was varied. Since the reactive groups were usedfor producing an In salt, the activity of the flux was reduced, so thatspheroid division of the molten alloy was not satisfactorily conducted.

[0169] [Effects of the Invention]

[0170] According to the material for a thermal fuse element and thethermal fuse of the invention, a small and thin alloy type thermal fusecan be provided in which a Bi—In—Sn alloy that does not contain a metalharmful to a living body is used as a fuse element, the operatingtemperature is 75 to 120° C., the initial operating characteristic ismaintained, and excellent heat cycle and aging resistances are attainedfor a long term.

[0171] According to the material for a thermal fuse element and thealloy type thermal fuse of claim 2 of the invention, since a fuseelement can be further thinned because of the excellent wire drawabilityof the material for a thermal fuse element, the thermal fuse can beadvantageously miniaturized and thinned. Even in the case where an alloytype thermal fuse is configured by bonding a fuse element to ato-be-bonded material which may originally exert an influence, a normaloperation can be assured while maintaining the performance of the fuseelement. Therefore, the thermal fuse is particularly useful as a thinthermoprotector for protecting a secondary battery which is requested tobe thinned because of attachment to a battery pack.

[0172] According to the alloy type thermal fuses of claims 3 to 10 ofthe invention, particularly, the above effects can be assured in a thinthermal fuse of the tape type, a thermal fuse of the cylindrical casetype, a thermal fuse of the substrate type, a thermal fuse having anelectric heating element, a thermal fuse or a thermal fuse having anelectric heating element in which lead conductors are plated by Sn, Ag,or the like, and a thermal fuse of the cylindrical case type in whichendsof the lead conductors have a disk-like shape, whereby theusefulness of such a thermal fuse can be further enhanced.

What is claimed is:
 1. A material for a thermal fuse element whereinsaid material has an alloy composition in which In is 15% or larger andsmaller than 37%, Sn is 5% or larger and 28% or smaller, and balance Bi,and in which, with respect to each of reference points of ternaryBi—In—Sn eutectic points of 57.5% Bi-25.2% In-17.3% Sn and 54.0%Bi-29.7% In-16.3% Sn, a range of ±2% Bi, +1% In, and ±1% Sn is excluded.2. A material for a thermal fuse element wherein 0.1 to 3.5 weight partsof one, or two or more elements selected from the group consisting ofAg, Au, Cu, Ni, Pd, Pt, Sb, Ga, and Ge are added to 100 weight parts ofan alloy composition of claim
 1. 3. An alloy type thermal fuse wherein amaterial for a thermal fuse element of claim 1 is used as a fuseelement.
 4. An alloy type thermal fuse wherein a material for a thermalfuse element of claim 2 is used as a fuse element.
 5. An alloy typethermal fuse according to claim 3, wherein said fuse element containsinevitable impurities.
 6. An alloy type thermal fuse according to claim4, wherein said fuse element contains inevitable impurities.
 7. An alloytype thermal fuse according to claim 3, wherein said fuse element isconnected between lead conductors, and at least a portion of each ofsaid lead conductors which is bonded to said fuse element is coveredwith a Sn or Ag film.
 8. An alloy type thermal fuse according to claim4, wherein said fuse element is connected between lead conductors, andat least a portion of each of said lead conductors which is bonded tosaid fuse element is covered with a Sn or Ag film.
 9. An alloy typethermal fuse according to claim 5, wherein said fuse element isconnected between lead conductors, and at least a portion of each ofsaid lead conductors which is bonded to said fuse element is coveredwith a Sn or Ag film.
 10. An alloy type thermal fuse according to claim6, wherein said fuse element is connected between lead conductors, andat least a portion of each of said lead conductors which is bonded tosaid fuse element is covered with a Sn or Ag film.
 11. An alloy typethermal fuse according to claim 3, wherein a pair of film electrodes areformed on a substrate by printing conductive paste containing metalparticles and a binder, said fuse element is connected between said filmelectrodes, and said metal particles are made of a material selectedfrom the group consisting of Ag, Ag-Pd, Ag-Pt, Au, Ni, and Cu.
 12. Analloy type thermal fuse according to claim 4, wherein a pair of filmelectrodes are formed on a substrate by printing conductive pastecontaining metal particles and a binder, said fuse element is connectedbetween said film electrodes, and said metal particles are made of amaterial selected from the group consisting of Ag, Ag-Pd, Ag-Pt, Au, Ni,and Cu.
 13. An alloy type thermal fuse according to claim 5, wherein apair of film electrodes are formed on a substrate by printing conductivepaste containing metal particles and a binder, said fuse element isconnected between said film electrodes, and said metal particles aremade of a material selected from the group consisting of Ag, Ag—Pd,Ag—Pt, Au, Ni, and Cu.
 14. An alloy type thermal fuse according to claim6, wherein a pair of film electrodes are formed on a substrate byprinting conductive paste containing metal particles and a binder, saidfuse element is connected between said film electrodes, and said metalparticles are made of a material selected from the group consisting ofAg, Ag-Pd, Ag-Pt, Au, Ni, and Cu.
 15. An alloy type thermal fuseaccording to claim 3, wherein a heating element for fusing off said fuseelement is additionally disposed.
 16. An alloy type thermal fuseaccording to claim 4, wherein a heating element for fusing off said fuseelement is additionally disposed.
 17. An alloy type thermal fuseaccording to claim 5, wherein a heating element for fusing off said fuseelement is additionally disposed.
 18. An alloy type thermal fuseaccording to claim 6, wherein a heating element for fusing off said fuseelement is additionally disposed.
 19. An alloy type thermal fuseaccording to claim 7, wherein a heating element for fusing off said fuseelement is additionally disposed.
 20. An alloy type thermal fuseaccording to claim 8, wherein a heating element for fusing off said fuseelement is additionally disposed.
 21. An alloy type thermal fuseaccording to claim 9, wherein a heating element for fusing off said fuseelement is additionally disposed.
 22. An alloy type thermal fuseaccording to claim 10, wherein a heating element for fusing off saidfuse element is additionally disposed.
 23. An alloy type thermal fuseaccording to claim 11, wherein a heating element for fusing off saidfuse element is additionally disposed.
 24. An alloy type thermal fuseaccording to claim 12, wherein a heating element for fusing off saidfuse element is additionally disposed.
 25. An alloy type thermal fuseaccording to claim 13, wherein a heating element for fusing off saidfuse element is additionally disposed.
 26. An alloy type thermal fuseaccording to claim 14, wherein a heating element for fusing off saidfuse element is additionally disposed.
 27. An alloy type thermal fuseaccording to claim 3, wherein said fuse element connected between a pairof lead conductors is sandwiched between insulating films.
 28. An alloytype thermal fuse according to claim 4, wherein said fuse elementconnected between a pair of lead conductors is sandwiched betweeninsulating films.
 29. An alloy type thermal fuse according to claim 5,wherein said fuse element connected between a pair of lead conductors issandwiched between insulating films.
 30. An alloy type thermal fuseaccording to claim 6, wherein said fuse element connected between a pairof lead conductors is sandwiched between insulating films.
 31. An alloytype thermal fuse according to claim 7, wherein said fuse elementconnected between a pair of lead conductors is sandwiched betweeninsulating films.
 32. An alloy type thermal fuse according to claim 8,wherein said fuse element connected between a pair of lead conductors issandwiched between insulating films.
 33. An alloy type thermal fuseaccording to claim 9, wherein said fuse element connected between a pairof lead conductors is sandwiched between insulating films.
 34. An alloytype thermal fuse according to claim 10, wherein said fuse elementconnected between a pair of lead conductors is sandwiched betweeninsulating films.
 35. An alloy type thermal fuse according to claim 11,wherein said fuse element connected between a pair of lead conductors issandwiched between insulating films.
 36. An alloy type thermal fuseaccording to claim 12, wherein said fuse element connected between apair of lead conductors is sandwiched between insulating films.
 37. Analloy type thermal fuse according to claim 13, wherein said fuse elementconnected between a pair of lead conductors is sandwiched betweeninsulating films.
 38. An alloy type thermal fuse according to claim 14,wherein said fuse element connected between a pair of lead conductors issandwiched between insulating films.
 39. An alloy type thermal fuseaccording to claim 3, wherein a pair of lead conductors are partlyexposed from one face of an insulating plate to another face, said fuseelement is connected to said lead conductor exposed portions, and saidother face of said insulating plate is covered with an insulatingmaterial.
 40. An alloy type thermal fuse according to claim 4, wherein apair of lead conductors are partly exposed from one face of aninsulating plate to another face, said fuse element is connected to saidlead conductor exposed portions, and said other face of said insulatingplate is covered with an insulating material.
 41. An alloy type thermalfuse according to claim 5, wherein a pair of lead conductors are partlyexposed from one face of an insulating plate to another face, said fuseelement is connected to said lead conductor exposed portions, and saidother face of said insulating plate is covered with an insulatingmaterial.
 42. An alloy type thermal fuse according to claim 6, wherein apair of lead conductors are partly exposed from one face of aninsulating plate to another face, said fuse element is connected to saidlead conductor exposed portions, and said other face of said insulatingplate is covered with an insulating material.
 43. An alloy type thermalfuse according to claim 7, wherein a pair of lead conductors are partlyexposed from one face of an insulating plate to another face, said fuseelement is connected to said lead conductor exposed portions, and saidother face of said insulating plate is covered with an insulatingmaterial.
 44. An alloy type thermal fuse according to claim 8, wherein apair of lead conductors are partly exposed from one face of aninsulating plate to another face, said fuse element is connected to saidlead conductor exposed portions, and said other face of said insulatingplate is covered with an insulating material.
 45. An alloy type thermalfuse according to claim 9, wherein a pair of lead conductors are partlyexposed from one face of an insulating plate to another face, said fuseelement is connected to said lead conductor exposed portions, and saidother face of said insulating plate is covered with an insulatingmaterial.
 46. An alloy type thermal fuse according to claim 10, whereina pair of lead conductors are partly exposed from one face of aninsulating plate to another face, said fuse element is connected to saidlead conductor exposed portions, and said other face of said insulatingplate is covered with an insulating material.
 47. An alloy type thermalfuse according to claim 11, wherein a pair of lead conductors are partlyexposed from one face of an insulating plate to another face, said fuseelement is connected to said lead conductor exposed portions, and saidother face of said insulating plate is covered with an insulatingmaterial.
 48. An alloy type thermal fuse according to claim 12, whereina pair of lead conductors are partly exposed from one face of aninsulating plate to another face, said fuse element is connected to saidlead conductor exposed portions, and said other face of said insulatingplate is covered with an insulating material.
 49. An alloy type thermalfuse according to claim 13, wherein a pair of lead conductors are partlyexposed from one face of an insulating plate to another face, said fuseelement is connected to said lead conductor exposed portions, and saidother face of said insulating plate is covered with an insulatingmaterial.
 50. An alloy type thermal fuse according to claim 14, whereina pair of lead conductors are partly exposed from one face of aninsulating plate to another face, said fuse element is connected to saidlead conductor exposed portions, and said other face of said insulatingplate is covered with an insulating material.
 51. An alloy type thermalfuse according to claim 3, wherein lead conductors are bonded to ends ofsaid fuse element, respectively, a flux is applied to said fuse element,said flux-applied fuse element is passed through a cylindrical case,gaps between ends of said cylindrical case and said lead conductors aresealingly closed, ends of said lead conductors have a disk-like shape,and ends of said fuse element are bonded to front faces of said disks.52. An alloy type thermal fuse according to claim 4, wherein leadconductors are bonded to ends of said fuse element, respectively, a fluxis applied to said fuse element, said flux-applied fuse element ispassed through a cylindrical case, gaps between ends of said cylindricalcase and said lead conductors are sealingly closed, ends of said leadconductors have a disk-like shape, and ends of said fuse element arebonded to front faces of said disks.
 53. An alloy type thermal fuseaccording to claim 5, wherein lead conductors are bonded to ends of saidfuse element, respectively, a flux is applied to said fuse element, saidflux-applied fuse element is passed through a cylindrical case, gapsbetween ends of said cylindrical case and said lead conductors aresealingly closed, ends of said lead conductors have a disk-like shape,and ends of said fuse element are bonded to front faces of said disks.54. An alloy type thermal fuse according to claim 6, wherein leadconductors are bonded to ends of said fuse element, respectively, a fluxis applied to said fuse element, said flux-applied fuse element ispassed through a cylindrical case, gaps between ends of said cylindricalcase and said lead conductors are sealingly closed, ends of said leadconductors have a disk-like shape, and ends of said fuse element arebonded to front faces of said disks.
 55. An alloy type thermal fuseaccording to claim 7, wherein lead conductors are bonded to ends of saidfuse element, respectively, a flux is applied to said fuse element, saidflux-applied fuse element is passed through a cylindrical case, gapsbetween ends of said cylindrical case and said lead conductors aresealingly closed, ends of said lead conductors have a disk-like shape,and ends of said fuse element are bonded to front faces of said disks.56. An alloy type thermal fuse according to claim 8, wherein leadconductors are bonded to ends of said fuse element, respectively, a fluxis applied to said fuse element, said flux-applied fuse element ispassed through a cylindrical case, gaps between ends of said cylindricalcase and said lead conductors are sealingly closed, ends of said leadconductors have a disk-like shape, and ends of said fuse element arebonded to front faces of said disks.
 57. An alloy type thermal fuseaccording to claim 9, wherein lead conductors are bonded to ends of saidfuse element, respectively, a flux is applied to said fuse element, saidflux-applied fuse element is passed through a cylindrical case, gapsbetween ends of said cylindrical case and said lead conductors aresealingly closed, ends of said lead conductors have a disk-like shape,and ends of said fuse element are bonded to front faces of said disks.58. An alloy type thermal fuse according to claim 10, wherein leadconductors are bonded to ends of said fuse element, respectively, a fluxis applied to said fuse element, said flux-applied fuse element ispassed through a cylindrical case, gaps between ends of said cylindricalcase and said lead conductors are sealingly closed, ends of said leadconductors have a disk-like shape, and ends of said fuse element arebonded to front faces of said disks.